The present invention relates to a method for controlling the cylinder charge in an internal combustion engine having variable timing of the gas-exchange valve of its cylinders. An internal recirculation of the residual gas in the cylinder is implemented by controlling the closing times of the at least one exhaust valve of the respective cylinder and by opening the at least one intake valve near top dead center, including the intermittent discharge of residual exhaust gas in front of the at least one intake valve.
Appropriate methods for exhaust control may include the early closing of all exhaust valves of a cylinder before the piston in the cylinder has reached its top dead center or the delayed closing of at least one exhaust valve of a cylinder only after at least one intake valve of the cylinder has opened. In the second case, a so-called valve overlap, a pressure drop from the exhaust side to the intake side is used for the internal recirculation of residual gas. This drop may be caused or intensified by the use of a throttle valve in the intake manifold.
In MTZ Motortechnische Zeitschrift 60 (1999) 7/80, pages 476-485, a choke-free load control including fully-variable valve gears for a spark-ignition engine is described. In engines having variable valve timing, the charge, or power output, is controlled, fully or in part, by controlling the opening curve of the gas-exchange valves (intake and exhaust valves of the cylinders). A throttle valve in the intake manifold may be present in addition, but may also be dispensed with entirely.
To be considered as variables of the opening curve of a valve, i.e., as parameters that may be adjusted by valve timing, are the following:
1. Beginning and end of the valve opening. This is also referred to as operating points for the valve opening. Generally, the operating points may be characterized by the crankshaft""s angular position relative to a, e.g., cylinder-specific, reference position. An important factor in this context may be the phase relation to the working cycle of the respective cylinder, such as the position during the compression cycle, expansion stroke, discharge stroke or intake stroke.
2. Valve lift.
3. The average or maximum velocity of the valve at opening or closing. This is also referred to as curve steepness of the valve lift curve.
In an engine having fully variable valve gears, as is described in the MTZ 60 (1999) 7/8, pages 479-485, the gas-exchange valves may be controlled directly and actuated by electromagnetic or electro-hydraulic valve actuators, for example. In particular, the cylinder charge may be metered solely through the appropriate controlling of the valves, the use of a throttle valve not being required. By reducing throttling losses, i.e., the pumping action of the engine, the engine efficiency may be increased and the specific consumption may thereby be reduced. Engines dethrottled in this manner may present a problem in that the conditions for the external mixture formation in the intake manifold may be more difficult. In engines controlled by throttle valves, a vacuum pressure may exist in the intake manifold, except in the case of a full load, which may allow a desirable vaporization of the fuel normally injected in the vicinity of the intake valves. In the case of a dethrottled engine, however, ambient pressure practically prevails in the intake manifold, causing a marked reduction in the vaporization rate and a corresponding increase in the fluid fuel portion (wall film). This may have a serious effect when the engine is cold, i.e., at cold start and during the subsequent warm-up phase. For instance, the control strategy of xe2x80x9cearly intakexe2x80x94closingxe2x80x9d which may be desirable in partial load operation when the engine is warm (cf. MTZ 60 (1999) 7/8) may turn out to be undesirable in a cold engine since fuel, partly in liquid form, may be drawn into the combustion chamber and not sufficiently processed, i.e., vaporized and homogenized together with the air, by the time ignition occurs. Moreover, by the early closing of the intake valves and due to the cooling of the cylinder charge during the subsequent expansion, fuel that has already evaporated may condense in the vicinity of bottom dead center of the piston. Under such conditions, the combustion quality, running smoothness, fuel consumption and the pollutant concentration in the exhaust gas may be correspondingly poor. In such timing of the intake valves, which is intended to reduce throttle losses to the greatest possible extent, these are opened near the piston""s top dead center and, given a partial load, are closed early, i.e., before bottom dead center.
With respect to the control strategy used for the gas-exchange valves, the following two conventional measures may improve the mixture formation and carburetion in a choke-free spark-ignition engine:
1. An intake-guided exhaust-gas recirculation, where residual gas is discharged via the intake valves and reaspirated. This internal recirculation of residual gas is described, for instance, in M. Pischinger, J. Hagen, W. Salber, T. Esch: Mxc3x6glichkeiten der ottomotorischen Prozessfxc3xchrung bei Verwendung des elektromechanischen Ventiltriebs, 7. Aachener Kolloquium Fahrzeugxe2x80x94und Motorentechnik, 1998, p. 987-1015.
2. Delayed opening of the intake valves, as it is described from the previously cited publication.
The two mentioned conventional methods may have the effect of lowering the pollutant concentration of the raw exhaust gases during warm-up of the engine. Furthermore, the method of internal recirculation of residual gas also may have this effect in engines at operating temperature. It may be important to reduce the pollutant concentration of the raw exhaust gas during the warm-up phase, such as, for example, in the early phase of warm-up during which the light-off temperature of the catalytic converter has not yet been reached and the catalytic converter may thus be only able to filter or convert the pollutants to a very limited degree.
By the strategy of a delayed opening of the intake valves, a vacuum pressure may be generated in the cylinder after the charge-change process, due to the expansion of the cylinder volume in top dead center. Intake valves are opened only once the pressure in the cylinder is sufficiently low, or once a supercritical pressure relationship between cylinder and intake manifold ( less than approx. 0.5) has been reached. This may result in a high inflow velocity (maximally sonic velocity) of the fresh gas into the respective cylinder. This highly dynamic, turbulent inflow-process may improve the mixture carburetion, namely due to the feature that liquid fuel portions are better atomized and are subsequently nearly completely evaporated. Moreover, the homogenization of the gas components air, fuel vapor and residual gas may be improved. In this manner, an improvement in the pollutant concentrations in the raw exhaust gas may be achieved, such as, for example, of unburned hydrocarbons (HC). Moreover, the improved combustionxe2x80x94apart from the achieved carburetion, the increased turbulence of the cylinder charge may also exert a positive influence herexe2x80x94may be able to mostly compensate for the increased charge-change losses of this method, such as in the case of a cold engine.
The other mentioned strategy may cause an increase in the portion of hot residual gas contained in the cylinder charge, for instance by closing the exhaust valves ahead of top dead center and the early opening of at least one intake valve, also before or near top dead center, optionally also by using a cylinder overlap. In addition, the pressure drop existing between cylinder and intake channel (intake manifold) in the vicinity of top dead center, given an already open inlet, may cause part of this hot exhaust gas to stream into the intake manifold. This residual gas, discharged in the vicinity of the fuel injector and fuel wall film, may cause a quick warming of the intake elbow after a cold start and may improve the mixture formation, i.e., the vaporization of liquid portions of the fuel in the intake manifold. While the intake valve remains open, the mixture of residual gas and fresh gas is then directly drawn into the cylinder. Due to an increased charge temperature, the further mixture carburetion in the cylinder may also be improved. This method of internal recirculation of residual gas may bring about a reduction in the fuel consumption as well as in the pollutant concentration of the raw exhaust gas (such as, for example, hydrocarbons). Moreover, the combustion temperature may be lowered due to charge dilution by residual gas, which in turn may significantly lower the pollutant concentration (NOx components) of the raw exhaust gas. Furthermore, a further decrease in the pumping work may improve the fuel consumption. On the other hand, a higher residual gas portion may also have a negative effect on the homogenization of the cylinder chargexe2x80x94in particular with respect to the components fresh gas and residual gas. Also, the combustion rate may go down and the inflammation phase may be lengthened when the residual-gas concentration rises. This may cause more cyclical combustion fluctuations, detrimentally affecting smooth running and combustion efficiency.
The present invention relates to a method that may ensure a desirable engine operation based on fuel consumption, pollutant emission and smooth running, in particular during the warm-up phase.
This may be achieved in that at least one intake valve is opened in the cylinder within a charge cycle, in at least two phases that are offset in time from one another. Moreover, an internal recirculation of the residual gas via the intake channel may be implemented by an appropriate control of the at least one exhaust valve and the at least one intake valve.
In addition to reducing the NOx concentration in the untreated exhaust gas, the controlling of the gas-exchange valves according to the present invention may achieve more efficient combustion and smoother running. The example method according to the present invention, due to a highly dynamic intake process following the second delayed opening of at least one intake valve and the resulting increased charge movement in the combustion chamber, may decrease or avoid the undesirable features for combustion stability and engine smoothness that are attributable to high portions of recirculated residual gas which, in the conventional methods of exhaust-gas recirculation, may be caused by a poor mixing of fresh gas and residual gas in the cylinder and a markedly slower combustion and which limit the achievable recirculation rate for the exhaust gas there. The example method according to the present invention may be implemented in an engine having fully variable valve timing without additional expenditure for components.
The example method according to the present invention may not only be used in a spark-ignition engine having manifold injection (external mixture formation) but also in engines having internal mixture formation, such as spark-ignition engines with direct injection, diesel engines.
Accordingly, at least one intake valve is opened, for example, in two phases that are offset in time from one another, the first phase of the intake-valve opening beginning in the vicinity of top dead center and the second phase of the intake-valve opening beginning after top dead center. The exhaust valves may be closed prior to the start of the first phase of the intake-valve opening or, if a valve overlap is used, may be closed prior to the end of the first phase of the intake-valve opening.
The beginning of the second phase of the intake-valve opening following the conclusion of the first phase may be selected such that a vacuum pressure builds in the cylinder to such a degree that a desired inflow velocity for the gas mixture into the cylinder may be achieved.
In the case of a 6-stroke or an 8-stroke engine operation, the second phase of the intake-valve opening may be offset by at least approximately one crankshaft rotation after the first phase of the intake-valve opening. This means that the residual gas quantity, which is recirculated after discharge of the residual gas into the area in front of the intake valves, remains in the intake manifold during the at least one piston rotation. In this case, the charge-cycle phase extends over more than two strokes, the at least two intake-valve opening phases that are offset in time from one another being separated by a particularly long period of time.
All intake and exhaust valves may be individually controllable with respect to lift, flank steepness, opening curve and opening times.
Furthermore, the beginning and the end of the opening phases and/or the lifts and/or the flank steepness of the opening curves for the intake and exhaust valves may be selected such that the respective desired quantities of fresh gas and residual gas are present in the cylinder after the charge-exchange process has been completed and that the inner flow of the combustion chamber as well as the turbulence of the cylinder charge are present in a desired form and intensity.
Moreover, the beginning and the end of the opening phases and/or the lifts and/or the flank steepness of the opening curves for the intake and exhaust valves may be controlled, based on the criteria of optimal mixture formation and exhaust-gas quality, the lowest possible energy input for the valve actuation and for the discharge and renewed aspiration of residual gas, as well as the lowest possible fuel consumption.